_respond_to_close never used

[Pithikos]

I'm not 100% sure this is a bug but I feel the above should have been used in the scenario below.

1. Server sends CLOSE.
2. Client sends CLOSE.
3. Server should close socket connection.

It seems from the code we never use _respond_to_close which should be used in this case. However I don't think it would work still since the server at step 1 has already poped every client and hence can't finalize the closing of the TCP socket.